This invention relates to brazed aluminum, and particularly to a process for improving the corrosion resistance of brazed aluminum products, as well as composite sheet materials for use in making such brazed products.
Aluminum has long been desired as a material of construction because of its resistance to corrosion. With the development of non-corrosive flux brazing and vacuum brazing techniques for joining aluminum parts, aluminum is now widely used in the manufacture of heat exchangers, particularly in the form of automotive radiators and evaporators for air conditioning units. Non-corrosive flux brazing and vacuum brazing have an important advantage over the traditional flux brazing in that they avoid the need for expensive cleaning operations normally associated with flux brazing.
Corrosion resistance is a concern common to components produced by all brazing methods. For example, road salt and moisture are sufficiently corrosive to often cause perforation of automotive radiators. In many cases corrosion problems originate in the brazing technique itself, e.g., through penetration of an element of the brazing alloy (particularly silicon) into the core alloy. This may occur along the grain boundaries of the core alloy during the brazing heat cycle and results in a sensitization of the core to intergranular attack.
It has been known for many years that corrosion problems in brazing can be alleviated to some extent by adding an interlayer between the structural member portion or core and the brazing layer. Such an interlayer is described, for instance, in Miller U.S. Pat. No. 2,821,014. The problem of corrosion in vacuum brazing is particularly considered in Singleton et al U.S. Pat. No. 3,788,824, where iron is added to either the core alloy or the cladding alloy to provide corrosion resistance and sag resistance. Various other patents such as U.S. Pat. Nos. 4,039,298, 4,093,782, 3,994,695, 4,339,510 and 4,649,087 describe various combinations of alloying components particularly for the core alloy which are claimed to provide benefit in intergranular corrosion resistance.
In Nakamura, U.S. Pat. No. 4,172,548, a procedure is described for controlling corrosion following vacuum brazing by controlling the grain size of the brazing sheet to at least 60 microns in diameter, this being achieved by a controlled cold work followed by a full anneal.
Finnegan et al, U.S. Pat. No. 4,586,964 also considers the problem of corrosion in a vacuum brazed produc and suggests a technique in which an interanneal is provided before a controlled cold work. This controlled cold work is maintained within a certain range of reduction.
With all of the above efforts to control corrosion in brazed products, corrosion still remains as a problem.